Stoker control



May 4', 1943. B. l PoNb 2,318,012

I STOKER CONTROL Filed Ot; 16, 1940 3 Sheets-Sheet 1 INVENTOR Beriram R Pom.

ATTORNEY May 4, 1943. B. P. POND 2,318,012

' STOKER CONTROL I Filed Oct. 16, 1940 3 Sheets-Sheet 2 I ATTORNEY STOKER CONTROL Fiied Oct. 16, 1940 3 Sheets-Sheet 3 INVENTOR 3 Berlram *P. Pond.

ATTORNEY UNITED STATES PATENT OFFICE s'roxsn CONTROL Bertram P. Ponds-River Forest, 111., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application October 16, 1940, Serial No. 361,427 14 Claims. (c1. 236-9) My invention relates to automatic controls for Another object is to provide a control arrangeiuel burning apparatus and is particularly conment for a fuel feeding device comprising a varcerned with controls embodying a combustion reiable resistance element exposed to combustion 'sponsive device for automatically operating temperature in combination with a heat actuated stokers to maintain fire under low heat requiredevice responsive to variations in resistance of ment conditions. the element.

In the past, efforts have been made to control Another object is to provide an arrangement stokers by means of temperature responsive deas set forth in the foregoing object wherein an vices having a temperature responsive element auxiliary heater is utilized in combination-with exposed to stack temperature or'temperature in 10 e hea t a e vi e to Pr d a h o the combustion chamber itself, it being recogticipating effect, that is to cause the device'to nized that fundamentally the proper way to constop the s o e s one t a it o y Wouldtrol a stoker for fire maintaining purposes is noth r O j t S' provide an arrang me t from the combustion temperature. Such devices, as set forth in the foregoing objects vwherein an while simple and inexpensive have been .found accumulating heater controlled by a space therobjectionable for various reasons, principally bemostat is-used in combination with the heat accause of rapid deterioration of the temperature u ted de ce to e ar y a se t e se t g o responsive element caused by h at, the fire maintaining apparatus to cause the stoker My invention comprehends the employment of to p r t t n ls d r n h r p n rman element having variable electrical resistance Derature after the theimostat is satisfied, i being exposed to temperature in the combustion chamr c ni d t at th s op ation mpr ves th mber of a fuel burning system such as a stoker no system, the resistance of the element varyin Another object is to provide an arrangement with combustion temperature, as set forth in the foregoing objects wherein the The variable resistance element preferably variable resistance element and device controlled trols a heat responsive switching device controlthereby m legs f a idge circuit ling the stoker, the actuation of the switchin Another object is to provide a bridge circuit ardevice being governed by a current fiow which is rangementhaving legs comprising heat responsive controlled by the resistance element, the resisti l n Cohtrflctible em pp n ance of which as pointed out depends on each other and actuating contacts depending upon bustion temperature. the relative energization of the elements; the rela My improved control arrangement as will readtil/e n a on of same being ntrolled by a ily be appreciated elimi at th ne of thning combustion temperature responsive resistance elemechanisms such as frequently employed in the mentcontrol of stokers, and avoids the use of temper- An Object s o pr vide means in the arature responsive elements which are subject to gement of the for oing object whereby aheat rapid deterioration when exposed to heat. The anticipating effect is produced when the stoker V particular combination of a variable electrical reis turned 1- sistance element and a heat actuated device is th r O j i to pr v d me ns ontrolled noteworthy in that relay coils are avoided, their 40 by the var a r sistan e lem n for shutt n use having the drawback that ther i s off the stoker in the event of no fire or high fire erable heating-of the coils which varies their reconditions; y additional ts; advantages, sistance. i and features of my invention will become ap- The primary object of my invention is to proparent from the following detailed description, vide a control arrangement for controlling fuel 4 nd n xed drawings wherein:

burning apparatus, principally stokers, utilizing Figure 1 is a stokersystem em-bodying a form mechanism not dependent on timers and not suby n n'fl h ject to the drawbacks inherent in ordinary de- Figure 2 is a similar system embodying a secvices such as temperatures responsive switches 1 nd form of my inventionvand when exposed to high temperatures. v Figure 3 is a similar system embodying a third Another object is to. provide a stoker controlform of my invention. o v I ling arrangement comprising a variable resist- I Referring to Figure 1 of the drawings wherein ance element exposed to combustion temperature a I have shown a first form of my invention, nu-

in combination with means responsive to the varmeral I llrepresents a stoker of conventional type,

iation in resistance of theelement. the stoker being driven by an electric motor and vice, that is,

.38 which carries a '36 and 31 cooperate with tacts 40 and 41 respectively.

having fuel feeding means H through which fuel is fed to a retort 12 disposed within a combustion chamber or space designated by the enclosure formed by the broken lines l3.

The control apparatus comprises a main relay 14 having a winding I with which is associated an armature which is attached to switch blades l6 and I1 and which moves the switch blades to the left into engagement with fixed electrical contacts 18 and I9 respectively when the winding I5 is energized.

Numeral 23 represents a space thermostat of conventional type having a circular bimetal element 24 carrying movable switch blades 25 and 26 which cooperate with fixed electrical contacts 21 and 28 respectively. Thermostat 23 is of a known type wherein blade 26 engages its associ ated contact at a fixed temperature and blade 25 engages its contact at another fixed temperature which may be one or two degrees lower, for example, the blade 26 being flexible to permit successive engagement of the blades with their contacts. Upon a rise in temperature the blades disengage from their respective contacts successively in reverseorder.

low. heat requirement conditions to maintain the fire, that is when the thermostat 23 remains satisfied for long periods, is represented by the.

element 30 disposed at a suitable point within the combustion space 13. The element 36 in the preferred form of the invention is a suitable length of wire, the resistance of which varies dependently upon the temperature within the combustion space. The element 36 may be any of various forms of wire, the resistance of which varies with temperature. Preferably a type of wire is chosen having such characteristics that its resistance changes appreciably with relatively small changes in temperature.

The element 39 controls a heat actuated deathermal relay designated by the numeral 3|. The relay or heat actuated'device 3| comprises two bimetal elements 32 and 33 both of which are attached to a pivoted member 34 which is pivoted on a pin 35, the bimetal elements-extending downwardly from the member 34. .The bimetal element 32 has attached to its lower end a contact strip 36 and carried by the strip 36 is another contact strip 31 and a bracket block 39. The contact strips fixed electrical con- Fastened to the biis a downwardly extending portion 43'extendmetal element 33 member 42 having an angular ing to the left. The member 42 is normally in engagement with a screw 44 carried by a fixed member 45. The left end of portion 43 is insu-. lated and carries a contact bracket 46 having a contact 41. Contact 41 cooperates with a contact 48 on a contact carrying bracket 49. The bracket 36 may be moved to the left by the heat actuated device so that its lower end engages the upper end of bracket 49 which extends above contact 46 as shown for separating the contacts "and 46 which are normally in engagement. Numeral 56 designates a permanent magnet which cooperates with the contact bracket 49 so as to tend to normally hold the contacts 41 and 46 in engagement and to cause them to operate with a snap action.

Adjacent the bimetal element 32 is a heating resistance 55 and adjacent the bimetal element 33 is a similar resistance 56. Heating of element 32 by resistance 55 tends to cause element 32 to warp in a direction moving strips 36 and 31 towards their respective fixed contacts. Heating of element 33 causes it to warp in a similar direction so that member 42 is urged more strongly against screw 44 so that when element 33 does warp, the pressure of member 42 against screw 44 rotates the heat actuated device as a whole aboutthe pivot 35, that is, in a clockwise direction. Thus if the elements 32 and 33 both warp in response to ambient temperature, that is, if the lower end of each is moved to the right the strips 36 and 31 will remain in the same position relative to their associated contacts because as element 32 warps towards the contacts the heat actuated device as a whole will be moved in a counter-clockwise direction moving them a similar amount away from their contacts. The arrangement of the bimetal elements on a pivoted member therefore acts to compensate for ambient temperature. that is, ambient temperature has no efiect on the control action of the device.

Numeral 51 designates a manually adjustable resistance connected in parallel with the heating resistance 56.

Power for the control apparatus is supplied by a step-down transformer 59 having a primary winding 59 and a secondary winding 60 having a fewer number of turns. The heating resistance 55 is at all times connected across the secondary 66 of transformer 58 and in series with theresistance element 30 by a circuit which is as follows: from secondary 60 through a wire 63, resistance I 55, wire 64, element 30, wire 65, and wire 66 back to secondary 66. Inasmuch as the resistance of element 36 varies dependently upon the temperature within the combustion space 13, the amount of current which flows through the circuit just described and consequently the amount of heat. givenofl by resistance depends upon the temperature within combustion space l3. Consequently, the heat which is imparted to bimetal element 32 and the position of contact strips 36 and 31 depends upon the last mentioned temperature. As the temperature within the combustion space 13 decreases, the resistance of element 30 also decreases, it having a positive coefflcient of resistance, and as it does so the amount, of current flowing in the circuit above described increases so as to increase the heating of element 32, tending to move the strips 36 and 31 closer to their associated contacts. With the parts in the position shown, the space thermostat is satisfied and the fire is diminishing inasmuch as the stoker is not operating. If the space thermostat remains satisfied until the fire has diminished to such an extent that a predetermined low temperature is reached in the combustion space 13, the heat given 011 by resistance 55 will increase to such an extent as to cause bimetal element 32 to' warp enough to bring contact strips 36 and 31 into engagement with contacts 46 and 41. Upon this occurring, a circuit is completed for relay l4 as follows: from secondary winding 66 through being from wire 68 through wire 15, wire 16,

resistance 56, and wire 11 back to wire 1|. The amount of current flowing through each of the two parallel branches will of course depend upon ture within the combustion space I3.

the adjustment of the manual resistance 61. Upon completion of the circuit described the relay I4 is energized and blades I6 and I1 are moved to the left into engagement with their associated contacts. Engagement of blade I6 withcontact I 6 completes a maintaining circuit for relay I4 as follows: from secondary 63 through wire 66, wire 61, contact bracket 43, contacts 46 and 41, contact bracket 46, wire 66, wire 69, variable resist- ,ance61, wire 13 to wire 1I (or from wire 66 through the parallel branch including resistance 66 to wire 1|), contact 43, contact strip 36, element 32, member 34, wire 13, wire 19, contact I6, switch blade I6, wire 66, windingv I6, and wire 14 back to secondary 66. It will be noted that this .last maintaining circuit is independent'of the contact strip 31 and contact 4| so that after the relay I4 is energized it remains energized until contact strip 36 is moved away from contact 46.

The contact strips 36 and 31 operate in sequence, the strip 36 first engaging contact 43 upon movement of element 32'to the right and the strip 31 then engaging contact 4|, and the contacts being disengaged in reverse order upon movement of the element 32 to thelett.

Upon movement of switch blade I1 into engagement with contact I3, 'a circuit for the stoker is completed from a line conductor. 63 through switch blade I1, contact I3, wire 8|, the stoker I3 back to line conductor. 62. After the stoker has been thus energized it continues in operation until the temperature within the combustion space I 3 has been raised to a predetermined value.

. As the stoker operates to feed fuel into the combustion space the temperature rises causing the resistance of element 33 to increase and thus to decrease the current flowing in heating resistance 66. Due to the reduced heating ofelement 32 it tends to warp to the left,that is, in a direction to move contact strips 36 and 31 away from their associated contacts. in temperature in the combustion space,- the strips 36 and 31.would be thus separated from their associated contacts. The purpose of the heating resistance-66 is to hasten this separation, that is, to produce a heat anticipating eil'ect so as to separate the strips 36 and 31- from their associated contacts sooner than they would be normally only as a result of therise .in tempera- Thus when relay I4 is energized and heating resistance 66 is energized the heating of element 33 causes its lower end to warp to the right moving member 42 more firmly against screw 44 soas to rotate the heat actuated device in a clockwise direction as described above, this action additionally tending to separate strips 36 and 31 from their asso' ciated contacts. Thus after the stoker has been in operation for a length of time and aiter there has been a rise in temperature in the combustion space I3, due to the reduced heating of element 32 and the heating of element 33, contact strips 36 and 31 will have moved away from their associ ated contacts so as to interrupt both the energizing circuit 01' relay I4 and the. maintaining circuit and to deenergize the heater 66. The amount of heat anticipation provided by heater 66 may be adjusted by adjusting the manual resistance 61. Theessential purpose of providing the heat anticipating effect is to prevent the fire maintaining operations, of the stoker-from being too long in the event'that the heat actuated After a predetermined rise device might not respond quickly enough to'the rise in temperature alone in the combustion space. I

In the event either element 33 or resistance as should burn outor otherwise fail bimetal elestrips 36 and 31 spaced from their associated contacts; In other words the apparatus will fail safe.

From the foregoing it will be understood that as long as the space thermostat 23 remains satisfied the stoker will be intermittently operated in response to the element 33 so as to maintain a fire in the combustion space l3. V

'In the event that there should be a call for heat from the space thermostat 23 such that theblades 26 and 26 are brought into engagement with their associated contacts a circuit for relay I4 is completed as follows: from secondary winding 66 through wire 66, wire 61, contact bracket 43, contacts 41 and 46, contact bracket 46, wire 63, wire 16, wire 33, contact 26, blade 26, blade 26, contact 21, wire 84, wire 13, winding I6, and wire 14 back to secondary 60. Completion of this circuit energizes the relay I4 and upon engagementof blade I6 with contact I3 a maintaining circuit for the winding I6 is completed as follows: from secondary 66, through wire 66, wire 61, contact bracket 43, contacts 41 and 46, contact bracket 46, wire 63, wire 16, wire'63, contact 26, blade 26, element 24,'wire 13, contact I8, blade I6, wire 66,.winding I6, and wire 14 back to secondary winding 66. By reason of this maintaining circuit the relay remains energized until the space temperature rises to a high enough value to sep- Engagement of blade I1 with contact I6 energizes the stoker as described above, the stoker of course operating as long the the relay is ener ized.

If during the time that the stoker is energized as a result of a call for heat-by the space thermostat 23 the temperature within the combustion space I3 should we to an unusually high value, the system may be shut down as a result 01' opening of contacts 41 and 48. Thus when the temperature afiecting element 30 rises to a predetermined high value, its resistance will be reduced to such an extent as to cause the amount of heating at the resistance 66 to be so reduced that element 32 by reason of the reduced heating .will warpto the left so as to bring arm 36 into engagement with contact bracket 43 so as to separate contacts 41 and 46. Disengagement of ping the stoker. Upon the ensuing reduction in temperature in the combustion space the contacts 41 and 46 may be reclosed as a result thereof, the stoker thus being cycled from the contacts 41 and 43 which form high fire or high limit contacts.

From the foregoing explanation of my invention those skilled in the art will appreciate its manifold advantages and its outstanding utility.- By its use the temperature in the combustion space is directly measured for purposes of con' trolling the stoker to maintain the fire when heating is not required. The use of a sensitive calibrated temperature responsive device subject to rapid deterioration is avoided. Moreover,

by use of the thermal relay the heating effect of the current through the control element 36 is utilized rather than this .heat becoming objectionable suchas might be "the case when using relay cofls or the like. 76

Referring to Figure 2 otthe drawings, I have than normally perature while it shown a somewhat modifiedfcrm of my invention. The form of Figure 2 however is similar in many respects-to that of Figure 1 and like numerals refer to like elements in these two figures. In Figure 2, power for the control apparatus is supplied by a step-down transformer I00 having a primary winding IN and a secondary 3 series across that portion of the secondary I02 to the left of the tap I03 by a circuit as'follows: from the left end of secondary I02 through wire I04, resistance 55, wire I05, resistance 55 and wire I08 back to tap I03. Inasmuch as the resistances 55 and 58 are at all times connected in series across the power source any variations in current will equally aflect the two heaters so thatsuch variations will be compensated for in the same manner as ambient temperature is compensated for. The element ,30 is connected in series with the lower half of resistance 55 across the left portion of secondary I02 by a circuit as follows: from the left end of secondary I02 through wire I01, wire I08, element 30, wire I09, the lower half of resistance 55, and wire I08 back to tap I03. By reason of thetap transformer I00, reduced power is used for the operation of the heat actuated device and thus the life of the element 30 and the heating resistances is prolonged. The element 30 controls the amount of heating at the lower half of resistance 55 and thus the bimetal element 32 operates the contacts associated therewith to control the relay and stoker in the same manner as in the previous embodiment.

In the present embodiment of the invention an additional heating resistance H5 is associated with the element 32 of the device 3|. The re-' sistance II5 forms what may be called an accumulating heater and if desired may be located within a heat insulating enclosure II8 adjacent the element 32. The heating resistance H5 is energized whenever the heater is energized by the space thermostat 23 and accumulates heat depending upon the length of time and the fre-' quency that the space themostat calls for heat.

The purpose of the heat supplied by resistance 1 H5 is to temporarily raise the control point of the fire maintaining mechanism, that is, the heat actuated device 3|. In other words, the

v heat supplied by the resistance I I5 will affect the element 32 causing it to assume a position wherein strips 38 and 31 are closer to their associated contacts than they are when resistance H5 is cold. Accordingly when element 32 is thus heated by resistance 5 less heat will be required from resistance 55 to cause strips 38 and 31 toengage contacts 40 and H than nor-' mally. In other words, the element 30 will cause operation of the stoker at higher temperatures depending upon the amount of heat which resistance II5 imparts to the element 32.- Thus after the space thermostat becomes satisfied thestoker will be intermittently operated during the drop in combustion temis falling to the temperature corresponding to the normal control point of the fire maintaining apparatus, that is, the stoker will be'operated by the fire maintaining apparatus at relatively higher'temperatures in the combustion chamber until the resistance 5 has cooled." The intermittent operation of the stoker during the drop in temperature after the thermostat is satisfied promotes efliciencyinasmuch as it causes fuel to be supplied at intervals to the central part of the firethus preventing the fire fromburning out first at the center, leaving a ring of unburned fuel around the outer portions of the retort. Thus when the thermostat 23 demands heat a circuit is completed for the relay I4 as follows: from secondary I02, through wire I01, wire IIO, contact bracket 49, contacts 41 and 48, contact bracket 48, wire III, wire II2, heating resistance II5, wire II3, contact 28, blade 28, blade 25, contact 21, wire II4, wire II5, winding I5, wire II8, back to secondary I02. Energization of relay I4 moves blades I8 and H to the left into engagement with their associated contacts and engagement of blade I8 with contact I8 completes a maintaining circuit for relay I4 as follows: from secondary I02 throughwire I01, wire IIO, contact bracket 49,

contacts 4'! and 48, contact bracket 45, wire III,

' I, contact I8, blade I8, wire II8, winding I5 and wire I I8 back to secondary I02. Thus it will be seen that whenever the relay I4 is energized by the thermostat 23 the heating resistance II5 isenergized. When the thermostat 23 becomes satisfied the heater H5 is also deenrgized but due to the heat which has accumulated the device 3| is subject to more heat than normal and thus its control point is temporarily raised and as pointed out above the fire maintaining operations will occur at higher than normal temperatures in the combustion space and the stoker will be intermittently operated at progressively lower combustionchamber temperatures as the combustion chamber temperature falls. Eventually, howeveflthe heater II5 will completely cool and the fire maintaining apparatus will operate at its normal control'point to maintain the desired temperature within the combustion space I3. The circuits by which the relay is energized for fire maintaining purposes may be traced as follows: from secondary I02, throughwire I01, wire IIO, contact bracket 49, contacts 41 and 48, contact bracket 48, wire III, wire II9, contact 40, contact strip 38, contact strip 31, contact 4|, wire I20, wire II5, winding I5, and wire II8 back be traced as follows: from secondary I02 through -wire I01, wire 0, contact bracket 43, contacts 41 and 48, contact bracket 46, wire III, wire II9, contact 40, contact strip 38, element 32, member 34, wire|2 I, wire II'I, contact I8, switch blade I8, wire II8, winding I5, and wire II8 back to secondary I02.

The advantages of the present embodiment lie in that the circuit arrangements of the heaters associated with the device 3| provide for compensation for current variations and prolong the life of the parts used. By use of the additional heating resistance II5 more efficient operation. is secured with very few and inexpensive additional parts. ure 1, the apparatus will fail safe in the event of burnout or failure of element 30 or resistances 55 and 58, bimetal element 32 when cool assuming a position with strips 38 and3'I spaced from contacts 40 and 4|. I 7

Referring to Figure 3 of the drawings, I have shown a third preferred form of the invention As in the embodiment of Fig-' Ventionthe heat actuated device 3i comprises a pivoted member I32 having an extending arm I33. Secured between the ends of member I32 on opposite sides of its pivot and fixed bracket members I34 and I35 is a pair of similar heat responsive elements I36 and I31. Each of the elements I36 and I31 is formed of a bimetal element formed into a minor spiral or helix which minor helix is then formed into a larger helix as shown. -This double helix arrangement results in linear motion of the elements upon a temperature change instead of rotary motion. The elements I36 and I31, as will presently be described, form legs ofa bridge circuit and are so formed as to shorten when heated. The brackets I34 and I35.also form electrical terminals.

The arm I33 forms a contact strip and it car-'- ries additional contact strips I 38 and I 39. The strip I33 cooperates with a contact screw I49 and the strips I38 and I39 cooperate with contact screws MI and I42, respectively, the screws ex;- tending through fixed members as shown. Contact strip I33 also cooperates with a contact I43 carried on the'end of a plunger I44, the plunger normally being urgedaway from the strip I 33 by a coil spring I45 and into engagement with the surface of a cam I46. The cam action. Adjacent the element I is an electrical .heating resistance I54.

Power for the control apparatus is supplied by a step-down transformer 51 having a primary winding 58 and a secondary winding 59, the secondarywinding having a tap 69.

As described above, the elements I66 and I91 form legs of a bridge circuit. In the present circuit the resistance element 39 is in series with the element I36'andthe resistance I49 is in series with the element I91. The circuit of element I36is as follows: from the tap 69 on secondary winding 69 through wire I6I, element I96, member I32, wire I62, wire I63, resistance element 39, wire I64, and wire ondary 59. The circuit for the element I91 is as follows: from tap 69 through wire I6I, wire I66, arm I48, resistance I49, wire I61, wire I62, member I32, element I31, wire I69, and wire I66 back to secondary 69.- The resistance element 39 may be located at a more or less remote point from the elements I36 and I31 so that the wires I69 and I64 may form relatively long lead lengths in series with the element I96 introducing a relatively large amount of additional resistance in circuit with element I 96. This additional resistance may undesirably aii'ect the characteristics of the bridge circuit and to compensate for this additional resistance that is, to balance it out the wire I61 may be connected to-point B instead of point A so that corresponding lengths'of wire will be in circuit with the element I91.

When the bridge circuit is balanced the parts may be substantially in the position shown, the currents in the legs of the bridge circuit being such as to cause the'heating of each of the ele- I 66 backto sec'-' ments I96 and I91 to be such that the forces which they produce balance each other, maintaining the member I92 in a static condition. Unbalancing of the bridge circuit by variation 5 in resistance of element 39 or adjustment of the arm I46 will vary the relative 'current fiow through the elements I36 and I31 so as to change the relative heating of each so that either one or the other exerts a preponderating force on the member I32v tending to cause it to rotate one way or the other about its pivot.

With the parts in the position shown, the thermostat 23 is satisfied and therefore the stoker is not operating and the fire is diminishing.

5 As the fire diminishes in size, the temperature in the combustion space I3 decreases corresponding'ly and thefresistance of element 39 also decreases, it having a positive coeflicient of resistance. When the temperature in combustion space I3 and correspondingly the resistance of element 39 decreases to a predetermined value, the relative current flow through element I36 and element 39 which is in series therewith increases so as to cause the heating of element I36 5 to preponderate over element I31. At the predetermined temperature value Just mentioned, element I36 will have shortened sufficiently due to additional heating to cause member I32 to rotate in a counter-clockwise direction bringing contact strips I38 and I39 into engagement with their respective contact screws I and I42.- 'I'he contact strips I 38 and I 39 are flexible and strip I38 engages screw I 4i slightly before strip I 39 engages screw I42. Strip I33 normally does not engage screw I49 but will engage screw I49 at a relatively low. temperature in the combustion space I3, indicating that the fire is extinguished. The function of strip I33 and screw I49 will be presently described. When contact strip I 39 eno gages screw I 42, a circuit is completed for-relay I4 as follows: from secondary 69 through wire "I, element I6I, contact I62, wire I12, winding I6, wire I13, contact screw I42, strip I39, arm- I93, member I92, element I91, wire I69, and wire I65 back to secondary 69. An-addition circuit is completed for winding I6 which is the same as the circuit Just described as far as member I92 but it then continues through element I96 and 'wire III back-to secondary 69. It will be noted 50 that in the low voltage bridge circuit through element I96 described above the current flow was from left to right as seen on Figure 3 and in the bridge circuit through element I91 the current fiow was from right to left. It will be seen that in the relay circuits traced the current flow is from right toleft in both of the elements I36 and I31. It will be seen therefore that the cur-' rent flows in the bridge circuit and in the relay circuit through element I31 are additivewhile these current flows through element I96 are in the opposite direction and tend to neutralize each other. Therefore, once been energized, due to the current flows through element I91 being additive, there be increased heating of element In which produce an anticipating eii'ect tending to move memherv I92 in a clockwise direction sooner than it would be in response onlyflto the increased temperature in the combustion space I9 resultingfrom stoker operation acting on resistance element 39. This anticipating eifect corresponds: to that produced in the modification of Figure 1 above. In the event of failure of element 99 due to burning out, the bridge circuit through element I96 would be interrupted. Element I36 after the relay I4 has a combustion space the would thus cool and lengthenthereby causing arm I33 to be moved in a clockwise direction toward contact,I43, the apparatus thus failing safe.

When the relay is energized, switch blades I6 and I1 are moved to the left into engage- -ment with their respective contacts. Engagement of blade I6 with contact I8 produces a maintaining circuit for relay I4 as follows: from secondary 53 through wire I, element I5I, contact I52, wire I12, winding I5, wire I14, wire I15, contact I8 blade I6, wire I16, screw I4I, contact strip I38, arm I33,-member I32, element I36, and wire I6I back to secondary 53. The maintaining circuit is also completed from secondary 53 through wire "I, element I5I, contact I52, wire I12, winding I5, wire I14, wire I15, contact I8, blade I6, wire I16, screw I4I,

' strip I38, arm I33, member I32,, element I31,

wire I63, and. wire I65 back to secondary 53.

Therefore, after the relay has been energized it will remain energized until member I32 is moved in a clockwise direction sufllciently to separate both strips I38 and I33 from screws I41 and I42. When blade I1 engages contact I3 a circuit is completed for thestoker as follows: from line conductor I11 through blade I1, contact I3, wire I18, the stoker I3 back to line conductor I13. The stoker continues in'operation as long 1 as the relay is energized.

move member I32 in a counter-clockwise direction enough to make strips I38 and I33 engage their associated contacts. In other words, adjustment of arm I48 in a counter-clockwise direction will lower the temperature which is maintained in the combustion space by the mechanism and movement of arm I48 in a clockwise direction will have the opposite effect.

The knob I41 has a normal settingwhich may beone to maintain a temperature of 200 degrees, for example. When the knob is at this setting, and if the temperature in the combustion space is substantially at a value corresponding to this setting, the portion of resistance I43 in circuit with element I31 is the same as the resistance of element 33; the resistance of elements I36 and I31 being the same, under these circumstances it will be seen that any variations in current at the power source will thus be compensated for, because, for the reasons just stated such variations will have the same effect on both of the elements I36 and I31.

The relay I4 may be energized for'starting the stoker by the space thermostat 23 when the temperature falls sufliciently to cause both blades 25 and 26 to engage their respective associated contacts. This circuit is as follows: from secondary 53 through wire "I, element I5I, contact I52, wire I12, winding I5, wire I14, wire I18, contact 21, blades 25 and 26, contact 28, wire I13, member I32, element I31, wire I63, and wire I65 back to secondary 53. When relay I4 is energized by this circuit, engagement of blade I6 with contact I8 completes a maintaining circuit as follows: from secondary 53 through wire III, element I5I, contact I52, wire I12, winding I5,

wire I15, contact I8, blade I6, wire I83, element 24, blade 26, contact 28, wire I13, member I32,

element I31, wire I63, and wire I65 back to secondary 53. .The' relay therefore remains energized until the space temperature rises to a high strips I38 and I33 will be disengaged from their 1 associated contact screws deenergizing the relay and stopping the stoker. The purpose of having the anticipating eilect is the same as in the' modification of Figure 1, that is, to hasten the stopping of the stoker after it has been started in response to the element 33 inasmuch-as it might be that the apparatus would not otherwise respond soon enough to heating of the element 33 for stopping the stoker,

As has been explained in the foregoing, the

' element 33 will actto intermittently start and stop the stoker, maintaining a fire in the combustion space when there are no demands for heat from the space thermostat. Normally the stoker will be thus started and stopped at predetermined temperatures depending upon the bridge circuit and of the element 33. To adjust the temperature which will be maintained in the bridge circuit characteristics may be varied by adjusting the knob I41 to move the arm I48 with respect to resistance I43. Thus if knob I" is adjusted in afldirection 50 as to reduce the part of resistance I43 which is in circuit with element I31, the relative current flow through element I31 and the heating thereof will tend to be increased so that it will tend to shorten,.moving member I32' in a clockwise direction. Thus it follows that the resistance of element 33 will have to be reduced to a relatively lower value before the current flow therethrough, and the heating of element I36 will be increased sufllciently to cause it to contact enou'gh value to cause blade 26 to separate from 28'. The stoker remains in operation as long as the relay remains energized in response to the space thermostat through a circuit which has already b een-described.'

If during the time that the stoker is. operating in response to the space thermostat the combustion space temperature should rise to an unduly high value,'the relay may be deenergized in response to the element 33. Thus when the temperature in the combustion space I3 reaches a predetermined relatively high value, the resistance of -element 33 will have increased to a relatively high value such as to so reduce the relative current flow therethrough and to reduce correspondingly the heating of element I36 so that due to its lengthening it preponderates over element I31, rotating member I32 in a clockwise direction bringing arm I33 into engagement with contact I43. Upon this occurring, a circuit is completed for heating resistance I54 as follows:

from.secondary 53 through wire I1I, resistance member 132, element m, wire I63, and-wire I back to secondary 53. The heat from resistance element I54=causes the bimetal element I 5I to warp to the left so as to disengage from contact I52 interrupting the relay circuits so as to deenergize the relay and stop the stoker. When the stoker has been stopped the combustion space temperature will begin to fall, thus tendingto cause the resistance of element 33 to decrease again. If the thermostat 23 remains in a posi- -tion calling for heat the reduction in temperature I54, wire I8I, plunger I44, contact I43, arm I33,-

in thecombustion space I3 may cause the relative heating of element I36 to increase again to an extent sufiicient to cause movement of member I 32, in acounter-clockwise direction moving arm I33 away from contact I43. This will deener'gize heating resistance I54permitting blade I5I ,to re-engage contact I52. If this occurs the thermostatic relay circuits will again be completed and the stoker willbe restarted and in this manner the stoker may be cycled in response to the element 30. From the foregoing, it can be seen that the contact I43 constitutes a high limit contact for shutting down the system under highfire conditions.

As has previously been described, the position of contact I43 with respect to arm I33 is adjustable by means of the manually rotatable cam I46. It will be seen that adjustment of .the knob I41 simultaneously adjusts the temsystem will be shut down under high fire conditions remain the same. cam I46 is such that when Thus the contour of knob I 41 is adjusted,

the position of contact I43 will be adjusted to just compensate for the variation in the bridge circuit characteristics occasioned by movement of arm, I48 in a manner to maintain the high fire shut-down setting constant. In other words,- when arm I48 is moved in a counter-clockwise direction so as to lower-the fire maintaining setting as described above, it tends to cause element I31 to shorten and move arm I33 towards contact I43. Thus this would tend to lowerthe high fire shutdown .setting as well because less lengthening of element I36 would then be required to bring arm I 33 into engagement with contact I43. counter-clockwise direction to-move arm I48 downwardly along resistance I49, cam I46 acts to move contact I43 downwardly a corresponding amount to maintain the high fire shutdown setting constant. Clockwise rotation of knob I41 of course moves arm I48 and cam I46 oppositely.

In the event that the fire in the combustion space I3 should go out entirely or diminish to such an extent that it would not recover upon the addition of fresh fuel thereto, the resistance of element 36 will drop to such an extent that theincreased heating and resulting shortening of element I36 will bring arm I33. into engage-' ment with contact screw I46 after strips I33 and I 39 have engaged their respective contact screws. Arm I33 will engage screw I40 at a predetermined relatively low temperature in the combustion space I3 and when it does so a circuit will be completed for heating resistance I54 as follows: from secondary 59 through wire "I,

- resistance I54, wire I8I, wire I82, contact screw} I32, element I31, wire I66, (0

I40, arm I33, member and wire -I65 back to secondary 59. En tion of heating resistance I54 will heat bimetal element I5I which willflex out of engagement with contact I5 2 interrupting So when knob" I4! is turned in a the relay circuits and shutting down the stoker as described above. 75

. propriate and convenient Figure 3, it would be in realm of my invention After the stoker 'has been thus shut down, the combustion space will c' tinue to cool and arm I33 will be held more 1 inengagement with contact screw I46 so that energized until fire has the combustion space. From the foregoing, those skilled in the art will appreciate that in the present form of the invention I have applied its principles in a novel manner. The present.embodiment has the advantages ascribed to the previous modifications and has the additional advantages inherent in bridge circuit arrangements. The arrangement provides all of the necessary functions with apadjustments.

It is to be understood that a bridge circuit similar to the one disclosed in Figure 3 might be used in combination with the heat actuated devices of the embodiments of Figures 1 and 2. It is also to be understood that it is within'the to utilize temperature responsive resistance elements having a negative coefllcient of resistance. .'If such an element were used, for instance in the form of that leg of the bridge circuit now occupied by the manually adjustable resistance.

The forms of my invention which I have disclosed are preferred forms and are understood to be exemplary. There are manyforms and skilled in the art and therefore the invention is not to be limited by my disclosure but only in accordance with the appended claims. I claim as my invention:

1. In a control system, in combination, a combustion chamber, an automatic stoker for feeding a heater associated with said thermal relay and controlled by said space'temperature responsive means for heating the relay to raise its control point during demands for heat whereby after the space temperature responsive means are satisfied the thermal relay acts in response to said element,

to intermittently operate said stoker while the combustion chamber temperature is falling, said heating means cooling during such time'allowing the thermal relay to gradually assume its normal control point.

2. In a system of the class described, in comtrol apparatus for the stoker ment. responsive to heat within the combustion chamber, said element carrying an electric carrent and being of such a nature that its resistance to current flow varies with said element'and electrical contacting means controlling the stoker operable in response to the relative energization of said other legs. saidother legs including expansible and contractible curthe relay cannot be rebeen reestablished in when the temperature in the comprising an elerent carrying elements associated with said contacting means for actuating same.

3. In a system of the class described in combination, an automatic stoker, a combustion chamber containing fuel fed by the striker, control apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying anelectric current and being or such a nature that its resistance to current flow varies with temperature, means formin a bridge circuit having said element-in one of its legs, said circuit having other legs the relative energizationoi which is dependent on said element, electrical contacting means controlling the chamber. said element carrying an electric current and being of such a nature that its resistance to current flow varies with temperature, means forming a bridge circuit having said ele- 'ment in one of its legs, said circuit having other legs the relative energization of which is dependent on said element, electrical contacting means controlling the stoker operable in response to the relative energization of said other legs,

variable resistance means controlling the relative'energization or said other legs to adjust the temperature at which said element brings about actuation of said contacting means to start the stoker, a thermostat controlling thestoker, means including an adjustable contact whereby said contacting means control .the stoker so as to stop it in response to a high temperature affecting said element, and ,means for simultaneously adjusting s'aid variable resistance and said adjustable contact, said adjusting means for the adjustable contact and variable resistance being so constructed and arranged that when the variable resistance is adjusted to vary the temperature at which the element starts the stoker, the adjust-' able contact is adjusted so as to maintain substantially constant the high temperature value v at which the stoker is stopped in response to said element.

5. In apparatus'of the character described, in combination, an automatic stoker, a combustion chamber containing'fuel fed by the stoker, control apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying anelectric current and being of such a nature that itsresistance tocurrent flow varies with temperature,and

means associated withsaid element responsive to the variations in current flow therethrough, said last means comprising a current carrying member adapted to generate heat when carrying current and being expansible and contractible dependently on the temperature resulting from said heat, and said last means-being a anged to start and stop the stoker. Y

6. In apparatus of the character described, in combination, an" automatic stoker, a combustion chamber containing fuel red by the stoker, control apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a nature that its resistance to current flow varies with temperature, and means associated with said element responsive to the variations in current flow therethrough, said last means comprising a pair of current carrying members adapted to generate heat when carrying current and being'expansible and contractible dependently on the temperature resulting from said heat, said element being arranged to vary the'relative current flows in said members and said members responding to said element to start and stop the stoker.

7. In a stoker of the class described, in combination, an automatic stoker, a combustion chamber containing fuel fed by the stoker, control apparatus for the stoker comprising. an element responsive to heat'within the combustion chamber, said element carrying an electric current and being of such a nature that its resistance to current flow varies with temperature, means forming a bridge circuit having said element in one of its legs, said circuit having other legs, the relative energization of which is dependent on said element, and electrical contacting means controlling the stoker operable in re sponse to the relative energization of said other legs. said, other legs comprising current carrying elements adapted to generate heat when carrying current and expansible and contractible dependently on the temperature resulting from said heat.

8. In a system of the class described, in combination, an automatic stoker, a combustion chamber containing fuel fed by the stoker, control apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a nature that its resistance to current flow varies with temperature,

means associated with said element responsive to the variations in current flow therethrough for normally starting the stoker at a relatively low temperature in the combustion chamber and for stopping the stoker after a relatively small rise in temperature, space thermostatic means controlling the stoker, and means associated with said element for stopping the stoker upon occurrence of a relatively low temperature in the combustion chamber indicating that the fire has gone out, said last means comprising a thermally actuated switch operable in response to said element.

9. In a control system, in combination, a combustion'chamber, an automatic stoker for feeding fuel to the chamber, control means comprising a thermal relay for controlling the stoker,

means comprising a temperature responsive resistance element exposed to heat in the combustion chamber, the resistance of said element varying as its temperature varies, said element bein associated with said relay to control its operation, said thermal relay and said element bein so related asvto start said stoker when the temperature in the combustion chamber is relatively low, and means controlled by said thermal relay and controlling its operation so as to cause the thermal relay to stop the stoker after it has been started sooner than it would normally be stopped in response to the resistance element,- said last means comprising a device for varyin the heating of the thermal relay.

'10. In a system of the class described, in combination, an automatic stoker, a combustion chamber containing fuel fed by the stoker, control apparatus for the stoker comprising an elemlnguponthesetting lsistance 9,818,012 merit responsive to heat within the combustion said last means controlling the stoker and comprising a heat responsive device the heating of which is dependent on the current flow through said element, space temperature responsive means controlling the stoker, and heating means con-- trolled thereby associated with said first mentioned means for modifying the control of the stoker by said first means.

11. In a system of the class described, in combination, fuel feeding means, a combustion chamber containing fuel fed by said fuel feeding means; first control means for said fuel feed ng means comprising an element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a nature that its electrical impedance varies with temperature, current responsive relay means associated with said element and responsive to' the variations in current flow therethrough, means operatively connecting said relay means to said fuel feeding means, and an adjustable resistance also associated with said relay means; said first control means being operative to control said fuel feeding means to maintain a predetermined temperature within the combustion chamber depending upon the setting of said adjustable resistance; means including a, further condition responsive device for controlling said fuel feeding means independently of said heat responsive element; said first control means being operativewhen a predetermined limiting temperature exists in said combustion chamber to terminate operation of said fuel feeding means independently of said further condition responsive device: and means to cause said last named operation to take place at the same limiting temperature responsive device for controlling said stolrer dependently of said heat first control means being operative when a predetermined. limiting temperature exists in said combustion chamber to terminate operation ofsaid stolrer independently of said further condition responsive device; and means including a compensating resistance adjusted simultaneously with said adjustable resistance to cause said last named operation to take place at the same limiting temperature regardless of the setting of said adjustable. resistance.

13. In a. control system, a combustion chamber. I I 7 fuel feeding means for feeding fuel to said chamber, an impedance element-located in said combustion chamber andbeing of a nature such that its electrical impedancevaries-with temperature,

relay means having a current sensitive element connected in' series with said variable impedance element, said relay means being connected to said fuel feeding means and controlling the op-' eration of the latter in accordance .with the amount of current flowing through said impedance and said current sensitive element, said relay mean and variable impedance element normally being operative to maintain a predetermined range of temperatures within the combustion chamber, and a space thermostatic switch predetermined high limit regardless of the setting of said adjustable resistancc.

sponsiverelaymeansassociatedwithsaidelement and responsive to the variations in current flow therethrough, means operatively connecting means;saidiirstconmeansbeing0pemtlv6 tocontrol the stoker tomaintain'a n mturewithinthechamberde- 0188111 re. m l ding a further condition uncircumcised .tion

associated with said relay means, and operative when the space feeding means even chamber temperature is above said predetermined range of values, said variable impedance element and relay means being operative when e combustion chamber temperature exceeds a eration of said fuel feeding means independently of said space thermostatic switch. C

14. In a controlsystem, a combustion chamber, fuel feeding means for feeding fuel to said chamber, relay means mperaturm gimtion of the heater of said thermal relay norto maintain a predetermined temperature chamber, a space condive switch controlling the energisetion of said relay means to cause operation of said fuel feeding means even though the-combustion chamber temperature is above said predetermined value, a further heater associated thermal relay, Im'ther heater and extent of sive switch inaccordancewiththefrequency timesaid'spacecondltionresponcalisfordeliveryoffueltovarytbe t mperature responsive element;

P. POND.

responsive element; said temperature drops below-a predetermined value to cause operation of said fuel though the combustion.

value to terminate opfor controlling the carer'- sensitive elementofsaid and 'meansfor g. said 

